1. Field of the Invention
The invention relates to an electric circuit configured to drive different xDSL signals, particularly ADSL and VDSL signals.
2. Description of the Prior Art
Various DSL (Digital Subscriber Line) standards have been developed in recent years. Examples of well known DSL standards are ADSL (Asymmetric Digital Subscriber Line), HDSL (High Bit Rate Digital Subscriber Line), IDSL (ISDN Digital Subscriber Line), MDSL (Medium Bit Rate Digital Subscriber Line), RDSL (Rate Adaptive ADSL), RADSL (Reverse ADSL), SDSL (Symmetric Digital Subscriber Line) and VDSL (Very High Speed Digital Subscriber Line).
Different DSL standards have different upstream and downstream data rates. Some DSL standards, for instance, ADSL, MDSL, SDSL and VDSL are used in combination with a conventional telephone service (POTS), while other DSL standards are not compatible with POTS, such as HDSL and IDSL. The xDSL standard which has been used most widely is the ADSL standard, which has different data transmission rates for the data streams flowing to the user (downstream) and for the data streams leaving the user (upstream). The ADSL standard is, therefore, also called asymmetric. For ADSL, data transmission rates are typically about ten times as high downstream, i.e. toward the user, as upstream, i.e. toward the service provider. For ADSL, the downstream data transmission rates are typically between 1 and 8 Mbps, while the upstream data transmission rates are between 100 and 800 kbps. The data transmission rates of VDSL are much greater and reach, for instance, 25 Mbps in both data transmission directions.
In the case of ADSL, digital multitone (DMT) modulated signals are transmitted in a relatively narrow frequency band with a frequency bandwidth of approximately 2 MHz. By contrast, the data transmission of VDSL takes place in a relatively wide frequency band with a frequency bandwidth of up to 30 MHz.
FIG. 1 shows an xDSL driver circuit based on the prior art. The xDSL driver circuit has a signal input for applying an xDSL signal which comes from an analogue front end (AFE). The applied xDSL signal is applied to the inputs of an operational amplifier OP via decoupling capacitors C1 and input resistors R1. The operational amplifier OP, which is a fully differential design in this exemplary embodiment, has two signal outputs which are each fed back via a feedback resistor R2 to the corresponding input of the operational amplifier OP. In addition, the outputs of the operational amplifier OP are coupled to primary windings L1 of a transformer TR via an output resistor R4. The transformer TR has secondary windings L2, with the ratio of the primary to the secondary windings resulting in a transformer ratio Ü. The transformer TR decouples a DC component from the output signal. A capacitor C2 is connected in series to the secondary windings L2, forming a high-pass filter for decoupling the POTS telephone signals from the data signals.
The outputs of the operational amplifier OP are also coupled crosswise via resistors R3 to the inputs of the operational amplifier OP. The resistors R3 are used for positive feedback in order to produce a synthesized output impedance for the driver circuit.
The output impedance ZA′ on the primary side of the transformer TR is obtained from the product of a synthesis factor, m, and the output resistor R4:ZA′=m 2R4.
The value of the feedback resistors R3 determines the synthesis impedance factor, m. The ratio of the resistor R2 to the resistance value of the input resistor R1 determines the signal gain or the gain G of the operational amplifier OP:G=R2/R1.
As the transformer ratio Ü increases, the supply voltage VDD required for the operational amplifier OP decreases. With an increasing transformer ratio Ü, however, the operational amplifier OP or line driver has to deliver a higher output current I in order to achieve the output power determined by the standard on the line.
The length of the signal or telephone lines between the xDSL driver circuit at the central station (central office) and the driver circuit at the user (customer premises) varies between different xDSL standards. Accordingly, the power P to be transmitted is likewise different for different xDSL standards. By way of example, the power P which is to be output onto the line is 20 dBm for ADSL 2+ and 14.5 dBm for VDSL. As mentioned before, for ASDL 2 the data are transmitted in a relatively narrow frequency band. For ADSL 2, the frequency bandwidth is 2.2 MHz. In this context, the data are transmitted in two separate subfrequency bands, with the first subfrequency band being provided for the data transmitted to the user (downstream) and the second subfrequency band being provided for the data transmitted from the user to the central station (upstream). A relatively small number of subfrequency bands or the small frequency bandwidth means that the risk of crosstalk in ADSL systems is relatively low. By contrast, for VDSL the data are transmitted in a relatively wide frequency band. VDSL1, for example, has a frequency bandwidth of 12 MHz and VDSL2, for instance, has a frequency bandwidth of 17 MHz. In this case, preferably three subfrequency bands are provided for the downstream data transmission direction and three subfrequency bands are provided for the upstream data transmission direction. The relative large transmission frequency bandwidth means that the risk of crosstalk is greater in VDSL systems than in ADSL systems.
In order to avoid the risk of crosstalk, the signals output by the VDSL driver circuit in a VDSL system are therefore transmitted at a lower power level than in ADSL systems. For VDSL, the prescribed maximum power level is 14.5 dBm, whereas in ADSL systems a maximum power level with the line of up to 20 dBm is admissible. The different signal powers mean that the voltage signal swing in the signal, which is output by the operational amplifier, is also different in different xDSL standards. The signal swing at the output of the operational amplifier is mainly determined by the supply voltage VDD applied to the operational amplifier. For ADSL systems, the supply voltage for the operational amplifier is 20 V, and for VDSL systems a typical VDSL driver circuit has a supply voltage of approximately 12 V.
The synthesized output impedance using the positive feedback resistors R3 minimizes the signal swing at the output of the operational amplifier and the latter's power consumption. As the frequency bandwidth for data transmission increases, the distortion caused by the operational amplifier increases due to the decreasing loop gain. Consequently, the synthesis factor, m, has an upper limit. The synthesis factor, m, in typical ADSL driver circuits is 5 to 6, whereas the synthesis factor in VDSL driver circuits is 3. The higher the transmission frequency bandwidth, the lower the admissible synthesis factor, m.
The signal swing at the output of the operational amplifier, for instance, is 17 Vp for an ADSL driver circuit at a supply voltage VDD=20 V, at a synthesis factor, m, of 6 and at a maximum permissible output power P of 20 dBm. By contrast, the maximum signal swing of the output of the operational amplifier for a VDSL driver circuit at a supply voltage of 12 V and a synthesis factor, m, 3 is 10 Vp, i.e. 10 V peak to peak.
The xDSL driver circuit based on prior art and being shown in FIG. 1 is thus configured in accordance to the xDSL standard. The table below shows the most important data for configuring xDSL driver circuits based on the prior art for the ADSL standard and the VDSL standard.
TABLEADSLVDSLSupply voltage VDD20 V12 VTransmission power P20 dBm14.5 dBmOutput resistor R4~5 Ω~10 ΩSynthesis factor m5-6 ~3       Gain    ⁢                  ⁢    G    =      R2    R1  ~16~11 Transformer ratio Ü1.31.3
A drawback of prior art xDSL driver circuits as shown in FIG. 1 is that different xDSL driver circuits need to be provided, depending on which type of xDSL signals are being transmitted. If a user wishes to change from ADSL to VDSL, for example, then the ADSL driver circuit needs to be replaced by a VDSL driver circuit, i.e. the hardware needs to be replaced. If the user wishes to return to the ADSL standard, then the VDSL driver circuit needs to be replaced by an ADSL driver circuit again, i.e. the boards are swapped again. This is naturally very laborious for the user. The xDSL driver circuit based on the prior art, as shown in FIG. 1, provides no kind of flexibility for the xDSL signal applied.